golang: Teach defer to chain exceptions (PEP 3134) even on Python2
Python3 chains exceptions, so that e.g. if exc1 is raised and, while it was not handled, another exc2 is raised, exc2 will be linked to exc1 via exc2.__context__ attribute and exc1 will be included into exc2 traceback printout. However many projects still use Python2 and there is no similar chaining functionality there. This way exc1 is completely lost. Since defer code is in our hands, we can teach it to implement exception chaining even on Python2 by carefully analyzing what happens in _GoFrame.__exit__(). Implementing chaining itself is relatively easy, but is only part of the story. Even if an exception is chained with its cause, but exception dump does not show the cause, the chaining will be practically useless. With this in mind this patches settles not only on implementing chaining itself, but on also giving a promise that chained cause exceptions will be included into traceback dumps as well. To realize this promise we adjust all exception dumping funcitons in traceback module and carefully install adjusted traceback.print_exception() into sys.excepthook. This amends python interactive sessions and programs run by python interpreter to include causes in exception dumps. "Careful" here means that we don't change sys.excepthook if on golang module load we see that sys.excepthook was already changed by some other module - e.g. due to IPython session running because IPython installs its own sys.excepthook. In such cases we don't install our sys.excepthook, but we also provide integration patches that add exception chaining support for traceback dump functionality in popular third-party software. The patches (currently for IPython and Pytest) are activated automatically, but only when/if corresponding software is imported and actually used. This should give practically good implementation of the promise - a user can now rely on seeing exception cause in traceback dump whatever way python programs are run. The implementation takes https://pypi.org/project/pep3134/ experience into account [1]. peak.utils.imports [2,3] is used to be notified when/if third-party module is imported. [1] https://github.com/9seconds/pep3134/ [2] https://pypi.org/project/Importing/ [3] http://peak.telecommunity.com/DevCenter/Importing This patch originally started as hacky workaround in wendelin.core because in wcfs tests I was frequently hitting situations, where exception raised by an assert was hidden by another exception raised in further generic teardown check. For example wcfs tests check that wcfs is unmounted after every test run [4] and if that fails it was hiding problems raised by an assert. As the result I was constantly guessing and adding code like [5] to find what was actually breaking. At some point I added hacky workaround for defer to print cause exception not to loose it [6]. [7] has more context and background discussion on this topic. [4] https://lab.nexedi.com/kirr/wendelin.core/blob/49e73a6d/wcfs/wcfs_test.py#L70 [5] https://lab.nexedi.com/kirr/wendelin.core/blob/49e73a6d/wcfs/wcfs_test.py#L853-857 [6] kirr/wendelin.core@c00d94c7 [7] zodbtools!13 (comment 81553) After this patch, on Python2 defer(cleanup1) defer(cleanup2) defer(cleanup3) ... is no longer just a syntatic sugar for try: try: try: ... finally: cleanup3() finally: cleanup2() finally: cleanup1()
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golang/_patch/__init__.py
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golang/_patch/ipython_py2.py
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golang/_patch/pytest_py2.py
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